Warmers for heating foods have been around for sometime, and generally consist of a low wattage heater element contained in a housing and operable to be turned on or off with a manual switch of some type. Sometimes an indicator light is illuminated when the heater is turned on. Some patents which have been issued on warmers for cups include U.S. Pat. No. 4,442,343 (Genuit et al., 1984); U.S. Pat. No. 4,523,083 (Hamilton, 1984); U.S. Pat. No. Des. 259,168 (Petrie, 1981); U.S. Pat. Des. 259,168 (Petrie, 1981) and British Design Registration 1,038,833 (Motor Electric Manufacturing Co., 1987). A number of coffee brewing systems are known having hot plates for maintaining a pot of coffee warm after it has been brewed. U.S. Pat. No. 2,660,658 (Wagner, et al., 1953) discloses an electric heating unit for heating a pressure cooker, and having a weight sensitive switch for actuating the unit only when the unit having the weight of a cooking vessel is installed on the machine. Many other units for keeping warm cooking vessels and plates are well known.
However, these units, and particularly units for keeping brewed coffee warm, have some serious problems. These problems relate to using these warmers when they are left unattended or when the user forgets to turn them off. If the liquid level in the vessel for holding the liquid is low, the evaporation can vaporize the liquid, leaving behind tars, solids and chemicals which were suspended in the liquid. These residues can cause stains, deterioration and potential damage to the containing vessel. In addition, if a paper towel or other flammable material is set over the warmer, it can be damaged or even set aflame. The longer the time such a material is on the warmer unit, the worse the degradation. When a coffee warmer is left on, with no vessel to warm or with an empty vessel in place, there is caused a waste of energy and possibly damage to the vessel. As discussed below, the present invention controls the warmer for a vessel for holding such liquids as brewed coffee, with a timing system for controlling the amount of time the warmer remains on, and further for controlling the operation of the unit when a vessel on the unit is removed from the unit and then replaced thereon. The foregoing aspect of the invention keeps the liquid warm yet avoids the overheating problems which have occurred in the prior art.